From: John Fastabend <john.fastabend@gmail.com>
To: David Ahern <dsahern@gmail.com>,
Jesper Dangaard Brouer <brouer@redhat.com>,
John Fastabend <john.fastabend@gmail.com>
Cc: bpf@vger.kernel.org, netdev@vger.kernel.org,
Daniel Borkmann <borkmann@iogearbox.net>,
Alexei Starovoitov <alexei.starovoitov@gmail.com>,
maze@google.com, lmb@cloudflare.com, shaun@tigera.io,
Lorenzo Bianconi <lorenzo@kernel.org>,
marek@cloudflare.com, Jakub Kicinski <kuba@kernel.org>,
eyal.birger@gmail.com
Subject: Re: [PATCH bpf-next V5 2/5] bpf: bpf_fib_lookup return MTU value as output when looked up
Date: Mon, 02 Nov 2020 08:18:55 -0800 [thread overview]
Message-ID: <5fa0316fc36a_1ecdb20851@john-XPS-13-9370.notmuch> (raw)
In-Reply-To: <dd0d1a41-1d1d-5104-0fa0-42241f5a960c@gmail.com>
David Ahern wrote:
> On 11/2/20 2:28 AM, Jesper Dangaard Brouer wrote:
> >>> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> >>> index e6ceac3f7d62..01b2b17c645a 100644
> >>> --- a/include/uapi/linux/bpf.h
> >>> +++ b/include/uapi/linux/bpf.h
> >>> @@ -2219,6 +2219,9 @@ union bpf_attr {
> >>> * * > 0 one of **BPF_FIB_LKUP_RET_** codes explaining why the
> >>> * packet is not forwarded or needs assist from full stack
> >>> *
> >>> + * If lookup fails with BPF_FIB_LKUP_RET_FRAG_NEEDED, then the MTU
> >>> + * was exceeded and result params->mtu contains the MTU.
> >>> + *
> >>
> >> Do we need to hide this behind a flag? It seems otherwise you might confuse
> >> users. I imagine on error we could reuse the params arg, but now we changed
> >> the tot_len value underneath them?
> >
> > The principle behind this bpf_fib_lookup helper, is that params (struct
> > bpf_fib_lookup) is used for both input and output (results). Almost
> > every field is change after the lookup. (For performance reasons this
> > is kept at 64 bytes (cache-line)) Thus, users of this helper already
> > expect/knows the contents of params have changed.
> >
>
> yes, that was done on purpose.
OK sounds good then. Thanks.
>
> Jesper: you should remove the '(if requested check_mtu)' comment in the
> documentation. That is an internal flag only -- xdp is true, tc is false.
next prev parent reply other threads:[~2020-11-02 16:19 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-30 16:50 [PATCH bpf-next V5 0/5] Subj: bpf: New approach for BPF MTU handling Jesper Dangaard Brouer
2020-10-30 16:50 ` [PATCH bpf-next V5 1/5] bpf: Remove MTU check in __bpf_skb_max_len Jesper Dangaard Brouer
2020-10-30 16:50 ` [PATCH bpf-next V5 2/5] bpf: bpf_fib_lookup return MTU value as output when looked up Jesper Dangaard Brouer
2020-10-30 19:40 ` John Fastabend
2020-11-02 9:28 ` Jesper Dangaard Brouer
2020-11-02 15:59 ` David Ahern
2020-11-02 16:18 ` John Fastabend [this message]
2020-10-31 15:52 ` David Ahern
2020-10-30 16:51 ` [PATCH bpf-next V5 3/5] bpf: add BPF-helper for MTU checking Jesper Dangaard Brouer
2020-10-30 20:23 ` John Fastabend
2020-11-02 11:15 ` Jesper Dangaard Brouer
2020-11-02 18:04 ` John Fastabend
2020-11-02 20:10 ` Jesper Dangaard Brouer
2020-11-12 12:58 ` Jesper Dangaard Brouer
2020-10-30 16:51 ` [PATCH bpf-next V5 4/5] bpf: drop MTU check when doing TC-BPF redirect to ingress Jesper Dangaard Brouer
2020-10-30 20:36 ` John Fastabend
2020-11-02 12:46 ` Jesper Dangaard Brouer
2020-11-02 16:23 ` John Fastabend
2020-10-30 16:51 ` [PATCH bpf-next V5 5/5] bpf: make it possible to identify BPF redirected SKBs Jesper Dangaard Brouer
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